1. Field of the Invention
The present invention relates to antenna systems for wireless communication devices, and more particularly to a simplified, low cost antenna system providing spatial diversity to combat multipath effects in communication systems.
2. Description of Related Art
In most wireless systems it is necessary to employ some form of antenna diversity to combat multipath effects in the communication system. The antenna diversity can be accomplished in the form of frequency diversity, time diversity, or spatial diversity. In frequency diversity, the system switches between frequencies to combat multipath interference. In the time diversity systems, the signal is transmitted or received at two different times, which works well in a rapidly changing environment. In spatial diversity systems, two or more antennas are placed at physically different locations combat multipath interference. The spatial diversity approach is probably the most common technique.
Many prior art systems in the 2.4 GHz ISM communications bands use a pair of ceramic patch antennas to form a spatially diverse antenna configuration. As shown in FIG. 1, a ceramic patch antenna comprises a ceramic substrate 10, a metalized patch 11 formed on one surface of the substrate 10, and a bottom side ground plane 13. A feed hole 12 couples the metalized patch 11 to the receiver/transmitter using the antenna as shown in FIG. 2. FIG. 2 is a side view of the ceramic patch antenna showing the metalized patch 11, the ceramic substrate 10, and the bottom ground plane 13. The feed hole 12 includes a solder fillet 14, connecting to a center wire of a coaxial cable 15, and also includes a solder fillet 16 which is coupled to the ground plane 13 and the shield of the coaxial cable 15. The use of high dielectric constant materials for the ceramic substrate 10 results in a set of antenna that are physically small. However, ceramic patch antennas tend to be relatively expensive. Furthermore, connecting the antenna to a low cost circuit board often requires special connectors and cabling, which also add cost to the system.
In addition, the ceramic patch antennas relying on high dielectric constant materials have a very high Q. This makes the antenna narrow band, and subject to manufacturing variations, and hence yield losses. Lastly, the patch antennas are directive with maximum radiation perpendicular to the face of the patch itself, with a null perpendicular to the ground plane. Having gain directivity is fine for locations where many patches can be used to cover all angles. However, it is best avoided in remote units where orientation of the antenna cannot be controlled.
One way to avoid the use of ceramic antennas is to fabricate the antenna on the same printed circuit board as the electronics. However, the board material has a much lower dielectric constant than ceramic, resulting in physically large antennas. Also, the configuration results in a difficult RF feed configuration, since the input of the antenna is on the bottom in the middle of the ground plane, making it difficult to connect a 50 ohm trace or other matched impedance lead, to the bottom side without breaking continuity on the ground plane.
An alternative which partially eliminates the feed arrangement problem is a printed dipole antenna on the board. One prior art example is shown in FIG. 3. The printed dipole includes first dipole element 20, a second dipole element 21, which are arranged to establish an antenna length .lambda./2, where .lambda. is the wavelength of the nominal center frequency for the antenna in free space. A balun 23 is required to implement an optimal feed arrangement between a 50 ohm unbalanced line 24, and the 75 ohm balanced dipole connection. The dipole antenna system is fabricated on the printed circuit board 25. The unbalanced 50 ohm line 24 is coupled to the transmitter power amplifier or into the receiver.
Another alternative in the prior art is to use physically small monopoles on the printed circuit board itself, even though prior art systems have not been matched for a 50 ohm standard feed. This approach requires a matching circuit which has a high Q. A high Q of the matching circuit results in a narrower bandwidth, as well as additional losses in the matching circuit itself.
Thus, the objects of a wireless antenna arrangement include the following:
1) The antennas are physically small.
2) The antenna can be fabricated on the same low cost circuit board as the rest of the electronics.
3) The bandwidth of the antenna is wide enough so that manufacturing tolerances will not cause degradation in performance.
4) The antenna is matched to 50 ohms so that the matching circuits are not required.
5) The feed for each antenna is a 50 ohm unbalanced line.
6) The antennas in the system are electrically isolated from each other, so that true spatial diversity can be achieved.